Method and apparatus for controlling a patient&#39;s body temperature in situ blood temperature modification

ABSTRACT

The present invention provides a method and apparatus for controlling the internal body temperature of a patient. According to the present invention, a catheter is inserted through an incision into a large blood vessel of a patient. By selectively heating or cooling a portion of the catheter lying within the blood vessel, heat may be transferred to or from blood flowing within the vessel and the patient&#39;s body temperature may thereby be increased or decreased as desired. The invention will find use in treating undesirable conditions of hypothermia and hyperthermia, or for inducing a condition of artificial hypothermia when desired. The method and system further provide for the cooling of initially hypothermic patients whose blood or body temperature has been warmed above the desired target level and the warming of initially hyperthermic patients whose blood or body temperature has been cooled below the desired target temperature.

[0001] The following application is a continuation-in-part of U.S. Pat.No. 5,486,208, dated Jan. 23, 1996 which was a continuation ofapplication Ser. No. 08/015,774 filed Feb. 10, 1993, now abandoned, thefull disclosures of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to the selectivemodification and control of a patient's body temperature. Moreparticularly, the present invention provides methods and apparatus fortreating hypothermia or hyperthermia by inserting a catheter into ablood vessel of the patient and selectively transferring heat to or fromblood flowing through the vessel.

[0004] 2. Description of the Background Art

[0005] Under ordinary circumstances the thermoregulatory system of thehuman body maintains a near constant temperature of about 37° C. (98.6°F.). Heat lost to the environment is precisely balanced by heat producedwithin the body.

[0006] Hypothermia is a condition of abnormally low body temperature.Hypothermia can be clinically defined as a core body temperature of 35°C. or less. Hypothermia is sometimes characterized further according toits severity. A body core temperature in the range from 32° C. to 35° C.is described as “mild” hypothermia, 30° C. to 32° C. is called“moderate,” 24° C. to 30° C. is described as “severe,” and a bodytemperature less than 24° C. constitutes “profound” hypothermia.Although the above ranges provide a useful basis for discussion, theyare not absolutes and definitions vary widely in the medical literature.

[0007] Accidental hypothermia results when heat loss to the environmentexceeds the body's ability to produce heat internally. In many cases,thermoregulation and heat production are normal but the patient becomeshypothermic due to overwhelming environmental cold stress. This is arelatively common condition, often resulting from exposure to theelements. Hypothermia may also occur in patients exposed to mild coldstress whose thermoregulatory ability has been lessened due to injury orillness. For example, this type of hypothermia sometimes occurs inpatients suffering from trauma or as a complication in patientsundergoing surgery.

[0008] Hypothermia of either type is a dangerous condition which canhave serious medical consequences. In particular, hypothermia interfereswith the ability of the heart to pump blood. Hypothermia may be fatalfor this reason alone. Additionally, low body temperature seriouslyinterferes with the enzymatic reactions necessary for blood clotting.This sometimes results in bleeding that is very difficult to control,even when normal clotting factor levels are present. These effects andother adverse consequences of hypothermia lead to drastically increasedmortality rates both among victims of trauma and in patients undergoingsurgery.

[0009] Simple methods for treating hypothermia have been known sincevery early times. Such methods include wrapping the patient in blankets,administering warm fluids by mouth, and immersing the patient in a warmwater bath. While these methods are very effective for mild hypothermia,more intrusive methods have been developed for treating severe andprofound cases of hypothermia. In particular, methods have been devisedto effect direct heating of a patient's blood. Most commonly, blood iswithdrawn from patient's circulation, passed through external warmingequipment, and reinfused back into the patient. Alternatively, the useof heated catheters has been proposed, where a catheter having a heatingelement near its distal end is inserted into the patient's vasculatureand heat directly transferred into the patient's circulating blood.

[0010] While the direct heating of patient blood can be highlyeffective, even in treating severe and profound cases of hypothermia, ithas been observed by the inventor herein that the excess transfer ofheat can cause the patient's temperature to rise above normal bodytemperature, resulting in hyperthermia. Hyperthermia can occur, forexample, when a hypothermic patient's metabolism begins to producesubstantial amounts of heat at the same time heat is being transferreddirectly to the blood.

[0011] It would therefore be desirable to provide methods for treatinghypothermia which further provide for treatment of accidental orincidental hyperthermia. In particular, it would be desirable to developsystems and methods for transferring heat to the blood where heat can beoptionally removed if the patient blood or body temperature exceeds atarget level. Such methods and devices will preferably employ a catheterfor direct heat transfer into circulating blood, but could also beuseful with methods where blood is heated externally from the patient'sbody. Such systems and methods should further be useful for thetreatment of patients who are initially hyperthermic, where the methodsand systems provide for initial cooling of the blood and optionalheating of the blood should the patient blood or body temperature fallbelow a target temperature.

SUMMARY OF THE INVENTION

[0012] The present invention provides apparatus and methods forrestoring normal body temperature in patients initially suffering fromhypothermia or hyperthermia. The apparatus includes a catheter and acontrol unit which together permit selective heating and cooling of thepatient's circulating blood. For hypothermic patients, the method willprovide for initially heating the blood until a target blood or bodytemperature has been restored. Heating will be stopped after reachingthe target temperature. Even after the heating has been stopped,however, the patient's blood and/or body temperature will continue to bemonitored to assure that the blood or body temperature does notovershoot the target. As discussed above, an initially hypothermicpatient can become hyperthermic if the total amount of heat experiencedfrom both patient metabolism and external heating exceeds that necessaryto restore normal body temperature. In the case of patients enteringhyperthermia, the method of the present invention provides for coolingthe patient's blood, usually using the same intravascular catheter orother apparatus which has been used for heating.

[0013] In the case of initially hyperthermic patients, the method of thepresent invention relies on cooling the patient's blood in order toreduce the blood and body temperature. Cooling will stop after a targettemperature has been reached. The patient's blood and/or bodytemperature will continue to be monitored, however, and should thepatient enter hypothermia, normal body temperature can then be restoredby introducing an appropriate amount of heat to the circulating blood.

[0014] According to a first aspect of the present invention, a systemfor restoring normal body temperature to a patient comprises anintravascular catheter having at least one heat transfer surface, atemperature sensor, and a control unit connectable to the temperaturesensor and the catheter. The control unit selectively transfers heat toor from the at least one heat transfer surface in order to achieve adesired target blood or body temperature. The intravascular catheter maycomprise a single heat transfer surface for both heat generating andheat absorption, but will usually comprise both a heat-generatingsurface and a separate heat-absorbing surface. The heat-generatingsurface will typically comprise a resistance heater, such as a wirecoil, and the heat-absorbing surface will typically comprise a metalfoil wrapped around the catheter, typically having an exposed area of atleast about 2 cm². In such cases, the control unit may comprise anelectrical current source connectable to the resistance heater and athermal electric cooler connectable to the metal foil. In an alternativeconstruction, the catheter may include at least one flow lumen whichpermits flow of a heat exchange medium within the catheter past the heattransfer surface. The control unit will then include a heater, a cooler,and a controller for selectively activating the heater or cooler totransfer heat to the heat exchange medium in order to restore normalbody temperature to the patient. The heater may be an electricalresistance heater and the cooler may be a thermoelectric cooler.

[0015] The temperature sensor will typically be on the catheter andmeasure the blood temperature. Alternatively or additionally,temperature sensor(s) may be separately attachable to the patient tomeasure body temperature.

[0016] In a second aspect of the present invention, a catheter forrestoring normal body temperature to a patient by selectivelytransferring heat to or from a patient's blood flow comprises a catheterbody having a proximal end and a distal end. The distal end isinsertable into a blood vessel, and the heat-generating heat exchangesurface and a heat-absorbing heat exchange surface are both disposednear the distal end of the catheter body. Typically, the catheter bodywill have a length in the range from about 15 cm to 50 cm and a diameterin the range from 1 mm to 5 mm. The heat-generating heat transfersurface will usually comprise an electrical resistance heater, and thecatheter will further comprise a connector which connects the resistanceheater to an external current source. The heat-absorbing heat transfersurface will typically comprise a metal foil wrapped around the catheterbody, and a heat-conductive element will extend through the catheterbody to near the proximal end to permit the heat-absorbing foil to beconnected to a cooler in a separate control unit. The metal foilheat-absorbing surface will typically have an area of at least 2 cm²,usually being from 4 cm² to 80 cm². The heat-conductive element could beeither a continuation of the metal foil surface (preferably beinginsulated in portions which will not lie within the blood circulation),or alternatively could be a metal core composed of a heat-conductivematerial.

[0017] According to the method of the present invention, normal bodytemperatures are restored to a patient by selectively introducing heatto the patient's blood flow for hypothermic patients or removing heatfrom the blood flow for hyperthermic patients. Usually, the heat will beintroduced or removed via an intravascular catheter which is connectedto an external control unit. Alternatively, the method of the presentinvention will also comprise the direct extracorporeal heating andcooling of the blood. A temperature characteristic of the patient ismonitored, typically being blood temperature and/or body temperature. Ifthe temperature characteristic indicates that initially hypothermicpatients have or are about to become hyperthermic, then heat will beremoved from the circulating blood to restore normal body temperature.Similarly, if the monitored temperature characteristic indicates thatinitially hypothermic patients are about to become hyperthermic, thenheat will be removed from the blood of those patients until normal bodytemperature has been restored.

[0018] The preferred intravascular catheters will be inserted into ablood vessel, usually being the femoral artery or vein, or the jugularartery or vein. The heat-introducing step comprises introducing heat ata rate between 10 W and 500 W, usually between 50 W and 250 W, while theheat removing step comprises removing heat at a rate from 1 W to 100 W.Preferably, the catheter and system described above will be employed.

[0019] For initially hypothermic patients, the temperaturecharacteristic will usually be blood temperature, and the target bloodtemperature, i.e., temperature at which heating is stopped, will be36.9° C. Should the blood temperature exceed 39° C., then cooling willcommence. For initially hyperthermic patients, the preferred temperaturecharacteristic will be blood temperature, and the target temperature atwhich cooling will be stopped will be about 36.9° C. Should the bloodcontinue to cool, typically to a temperature of 36° C. or below, thenblood heating will commence. Is should be appreciated, however, thatthese temperature targets are nominal objectives, and the methods of thepresent invention can be practiced with target temperatures which differsomewhat from those just set forth.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 depicts a catheter according to the present inventioninserted percutaneously into a blood vessel of a patient;

[0021]FIG. 2 depicts a catheter suitable for increasing the temperatureof a patient's blood by electrical resistance heating;

[0022]FIG. 3 depicts the distal end of a catheter having a resistanceheating element and a temperature sensor;

[0023]FIG. 4 depicts the distal end of a catheter having an optical waveguide and an optical diffusing tip for converting laser energy intoheat;

[0024]FIG. 5 depicts a catheter in which heat is transferred down athermally conductive shaft between the distal end of the catheter andheating or cooling apparatus at the proximal end of the shaft;

[0025]FIG. 6 depicts a catheter in which a heated or cooled fluid flowsthrough a balloon, which provides for an increased surface area at thedistal end;

[0026]FIG. 7 depicts a catheter having a resistance heating element atits distal end and a balloon having longitudinal ribs to furtherincrease the heat transfer surface area;

[0027]FIG. 8A depicts a catheter having longitudinal fins at the distalend of the catheter body;

[0028]FIG. 8B depicts a catheter having radial ribs at the distal end ofthe catheter body; and

[0029]FIG. 8C depicts a catheter having a spiral fin to increase theheat transfer area at the distal end of the catheter.

[0030]FIG. 9 illustrates a catheter having a resistance heater whichheats a fluid filling a balloon. Current flows through the fluid from apair of conduction wires embedded in the catheter body.

[0031]FIG. 10 illustrates the control schemes for raising bodytemperature in a patient suffering from hypothermia and lowering bodytemperature in a patient suffering from hyperthermia, respectively.

[0032]FIG. 11 illustrates a preferred catheter for the selective heatingand cooling of patient blood flow employing a wire coil resistanceheater and a metal foil cooling element.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033] The present invention provides methods and apparatus forselectively modifying and controlling a patient's body temperature bywarming or cooling the patient's blood, usually using an intravascularcatheter in situ. According to the preferred method of the presentinvention, the catheter is inserted through a puncture or incision intoa blood vessel in the patient's body. By warming or cooling a portion ofthe catheter, heat may be transferred to or from blood flowing withinthe vessel and the patient's body temperature may thereby be increasedor decreased as desired. During the procedure, the patient's bloodand/or body core temperature may be independently monitored andtreatment may continue until the patient's blood and/or body coretemperature approaches the desired level, usually the normal bodytemperature of about 37° C. Such methods will find use in treatingundesirable conditions of hypothermia and hyperthermia and may also beused to induce an artificial condition of hypothermia when desired,e.g., to temporarily reduce a patient's need for oxygen. In such a case,the patient's temperature may be reduced several degrees Celsius belowthe normal body temperature.

[0034] In treating conditions of hypothermia and hyperthermia there isthe possibility that the patient's core body temperature will“overshoot” the target normal body temperature. The body's metabolicresponse to the external heating or cooling being applied, as describedabove, can result in overcompensation of the initial condition. Inparticular, when heating the patient's body to treat hypothermia, thebody's own heat generation arising from internal metabolic processes mayraise the body temperature in an unpredictable manner, resulting in abody temperature that can rise well above normal body temperature. Insuch cases, the present invention provides for a reversal of thetransfer of heat from or to the patient's blood. In the case of anuncontrolled temperature rise, the system of the present invention willbe switched so that heat will be withdrawn from the circulating blood.Conversely, in the case of overcooling of the patient's body, the systemwill be switched so that heat will be introduced to the patient.

[0035]FIG. 1 depicts a distal end 15 of a catheter 10 according to thepresent invention. The catheter has been inserted through the patient'sskin into a blood vessel BV. Blood flow through the vessel is indicatedby a set of flow arrows F. Preferably, the catheter will be insertedinto a relatively large blood vessel, e.g., the femoral artery or veinor the jugular vein. Use of these vessels is advantageous in that theyare readily accessible, provide safe and convenient insertion sites, andhave relatively large volumes of blood flowing through them. In general,large blood flow rates facilitate quicker heat transfer into or out ofthe patient.

[0036] For example, the jugular vein may have a diameter of about 22French, or a bit more than 7 millimeters (1 French=0.013 inches=0.33mm). A catheter suitable for insertion into a vessel of this size can bemade quite large relative to catheters intended for insertion into otherregions of the vascular system. Atherectomy or balloon angioplastycatheters are sometimes used to clear blockages from the coronary arteryand similar vessels. These catheters commonly have external diameters inthe range between 2 and 8 French.

[0037] In contrast, it is anticipated that a catheter according to thepresent invention will typically have an external diameter of about 10French or more, although this dimension may obviously be varied a greatdeal without departing from the basic principles of the claimedinvention. It is desirable that the catheter be small enough so that thepuncture site can be entered using the percutaneous Seldinger technique,a technique well known to medical practitioners. To avoid vessel trauma,the catheter will usually be less than 12 French in diameter uponinsertion. Once in the vessel however, the distal or working end of thecatheter can be expanded to any size so long as blood flow is not undulyimpeded.

[0038] Additionally, the femoral artery and vein and the jugular veinare all relatively long and straight blood vessels. This will allow forthe convenient insertion of a catheter having a temperature controlledregion of considerable length. This is of course advantageous in thatmore heat may be transferred at a given temperature for a catheter of agiven diameter if the length of the heat transfer region is increased.

[0039] Techniques for inserting catheters into the above mentioned bloodvessels are well known among medical personnel. Although the method ofthe present invention will probably be most commonly employed in ahospital, the procedure need not be performed in an operating room. Theapparatus and procedure are so simple that the catheter may be insertedand treatment may begin in some cases even in an ambulance or in thefield.

[0040] The distal end 15 of the catheter may be heated or cooled asdesired and held at a temperature either somewhat above or somewhatbelow the patient's body temperature. Blood flowing through the vesselwill thereby be warmed or cooled. That blood will be circulated rapidlythroughout the patient's circulatory system. The beneficial effect ofwarming or cooling the patient's blood in the vicinity of the catheterwill thereby be spread very quickly throughout the entire body of thepatient.

[0041]FIGS. 2 and 3 depict a catheter suitable for treating hypothermiaby increasing the temperature of a patient's blood. As depicted in FIG.2, the catheter has a preferably flexible catheter body 20. Disposedwithin the catheter body are a pair of electrical conduction leads 22and 23 and a temperature measurement lead 25.

[0042] Electrical conduction leads 22 and 23 are connected to aresistance heating element 28, as depicted in FIG. 3. Electrical currentprovided by a power source (not shown) is converted to heat within theheating coil. That heat warms distal end 15 of the catheter and isthereby transferred to blood flowing through the vessel.

[0043] Temperature measurement lead 25 is connected to a temperaturesensor 30. The temperature sensor facilitates the control of currentflow through the heating coil. It is important to closely monitor thetemperature of the distal end of the catheter and thus the flow of heatinto the patient's blood. Care must be taken not to overheat the bloodwhile still providing an adequate rate of heat transfer into thepatient. The provision of a sensitive temperature sensor at the distalend of the catheter will help to achieve this goal.

[0044]FIG. 4 depicts an alternate embodiment of a catheter having meansfor transferring energy from an external power source to distal end 15of catheter body 20. In this embodiment, laser energy from a laser lightsource (not shown) is transmitted along optical wave guide 35. The waveguide directs the laser energy into optical diffusing tip 37, whichconverts the laser energy to heat. From diffusing tip 37, the heatradiates outward into distal end 15 of the catheter and from there intothe patient's blood stream.

[0045]FIG. 5 depicts another catheter suitable for practicing thepresent invention. This embodiment has a thermally conductive shaft 40running the length of catheter body 20. Shaft 40 is made of a metal orother material having a high thermal conductivity. By heating or coolingthe proximal end 42 of shaft 40 with an external heating or coolingapparatus 45, heat will be caused to flow either into or out of thedistal end 47 of the shaft. In the embodiment depicted, the distal endof the shaft is fitted with heat transfer vanes 50, which add to thesurface area of the shaft and thereby promote more effective heattransfer between the catheter and the patient's blood stream.

[0046]FIG. 6 depicts still another means for transferring heat to orfrom the distal end of a catheter. In this embodiment, catheter body 20has two lumens running through it. Fluid flows from the proximal end ofthe catheter through in-flow lumen 60, through a heat transfer region62, and back out through out-flow lumen 64. By supplying either warmedor cooled fluid through inflow lumen 60, heat may be transferred eitherto or from the patient's blood stream.

[0047] In the embodiment depicted, heat transfer region 62 is in theform of a balloon 70. Use of a balloon will be advantageous in someembodiments to provide an increased surface area through which heattransfer may take place. Balloon inflation is maintained by a pressuredifference in the fluid as it flows through in-flow lumen 60 andout-flow lumen 64. The balloon should be inflated to a diameter somewhatless than that of the inside diameter of the blood vessel so as not tounduly impede the flow of blood through the vessel.

[0048]FIG. 7 depicts a catheter having an internal resistance heatingelement 28 and a balloon 70, which is shown inflated. In thisembodiment, the increased surface area provided by the inflated balloonis further augmented by the presence of a set of longitudinal fins 75 onthe surface of the balloon. Alternatively, longitudinal fins 75, radialribs 77, or one or more spiral fins 79 may be disposed directly on thebody 20 of a catheter as shown in FIGS. 8A, 8B and 8C. Ordinarily,longitudinal ribs will be most advantageous because they restrict bloodflow through the vessel less than other configurations. In fact, theseribs insure that the balloon will not block the flow of blood throughthe vessel because a flow path will always be maintained (between theribs) regardless of how much the balloon is inflated.

[0049] Inclusion of a balloon on a catheter employing resistance heatingallows for designs in which current is conducted through the fluid whichfills the balloon. The catheter depicted in FIG. 9 has a catheter body20 about which is disposed an inflatable balloon 70. The balloon isinflated by injecting a suitable fluid into the balloon through centralballoon inflation lumen 80. In this embodiment, current flows from anexternal source of electrical power (not shown) through conduction wires82 and 84 to electrodes 86 and 88.

[0050] A suitable fluid will allow current to flow between electrodes 86and 88. Common saline solution, for example, contains dissolved ionswhich can serve as charge conductors. Electrical resistance within thefluid will cause the fluid to be heated, thus providing the desiredwarming of the catheter. The amount of warming will be dependant uponthe voltage between the electrodes, the distance between them, and theresistivity of the fluid. The relation between these quantities isfairly simple; one skilled in the art will have no difficulty selectingappropriate values.

[0051] Resistance heating catheters like those depicted in FIGS. 3, 7and 9 may use DC or low frequency AC power supplies. However, it may bedesirable to use a higher frequency power supply. For example, it isknown that the risk of adverse physiological response or electrocutionresponse may be lessened at frequencies within the range of about 100kilohertz to 1 megahertz. Power supplies that operate at thesefrequencies are commonly referred to as radio-frequency, or RF, powersupplies.

[0052] A catheter according to the present invention should be designedto optimize the rate of heat transfer between the catheter and bloodflowing through the vessel. While a large surface area is desirable inorder to maximize heat transfer, care must be taken so that the catheterdoes not unduly restrict blood flow through the vessel. Furthermore, thetemperature of the catheter should be carefully controlled to preventundesirable chemical changes within the blood. This is especiallyimportant when applying heat to the blood as blood is readily denaturedby even moderately high temperatures. The exterior temperature of acatheter for warming blood should generally not exceed about 42° C. -43°C.

[0053] It is estimated that a catheter whose surface temperature iscontrolled between 37° C. and 42° C. will provide a body core warmingrate of approximately one to two degrees Celsius per hour in a patientstarting out with severe hypothermia. This estimate is highly dependanton a number of factors including the rate of blood flow through thevessel, the initial body temperature of the patient, the externalsurface area of the catheter through which heat is conducted, etc. Theactual rate achieved may vary substantially from the above estimate.

[0054] The above estimate provides a starting point for a rough estimateas to the level of power transferred from the catheter to the patient'sbody and therefore of the size of the power supply required by thesystem. Regardless of the exact means of power transmission chosen,resistance heating coil, laser and diffusing tip, direct conduction orfluid circulation, an appropriate power supply will be required toprovide heat to the system.

[0055] The sum of heat entering and leaving a patient's body can bewritten as:

ΔH=H _(c) +H _(i) −H _(e)

[0056] where ΔH is the sum of all heat transferred, H_(c) is the heattransferred from the catheter to the patient, H_(i) the heat produced bythe patient internally, and He the heat lost from the patient to theenvironment. If one assumes, as will ordinarily be the case in a healthypatient, that the body's internal thermoregulatory system will producejust enough heat to offset heat lost to the environment, then theequation is made simple:

ΔH=H _(c).

[0057] The above equation can be written in terms of the change in thepatient's internal body temperature over time as follows:

mc(ΔT/Δt)=(ΔH _(c) /Δt)

[0058] where m is the body mass of the patient, c is the specific heatof the patient's body, (ΔT/Δt) is the time rate of change of thepatient's internal body temperature, (ΔH_(c)/Δt) is the time rate ofheat delivery from the catheter to the patient.

[0059] If one assumes a patient having a body mass of 75 kilograms and aspecific heat of 4186 joules/° C.-kg (assumes the specific heat of thehuman body to be the same as that of water, the actual value will besomewhat different), then a warming rate of 1° C. per hour (3600seconds) will require the catheter to transfer heat to the patient at arate of about 87 watts (1 watt=1 joule/sec).

[0060] However, as an estimate of the desirable size of a power supplyto be used with a catheter of the present invention, this estimate isalmost certainly too low. This is true for a number of reasons. First,it was assumed for the sake of convenience that the patient's internalsystem would produce an amount of heat equal to that lost to theenvironment. In a hypothermic patient this will obviously not be thecase. Almost by definition, hypothermia occurs when a person's abilityto produce heat internally is overwhelmed by heat lost to theenvironment. The catheter will have to make up the difference so thepower level required will need to be greater for that reason alone.

[0061] Additionally, the above estimate does not allow for power lossesbetween the power supply and whatever warming means is utilized. Suchlosses could include resistance losses in electrical transmission linesbetween the power supply and a resistance heating element, inherentinefficiencies and other losses in a system having a laser and adiffusing tip, heat losses along a thermally conductive shaft or fluidcirculation lumen, and the like. Any such losses which do occur willneed to be compensated for by additional power supply capacity.

[0062] Furthermore, it would be undesirable to limit the performance ofa catheter according to the present invention by limiting the size ofthe power supply used. It would be preferable instead to use a powersupply capable of providing power considerably in excess of thatactually needed and then controlling the delivery of that poweraccording to the measured temperature of the catheter itself. Asmentioned previously, this can be readily accomplished by including asensitive temperature sensor within the body of the catheter.Nevertheless, the above calculation can be used as a useful estimate ofthe likely lower bound for sizing a power supply for use in a catheteraccording to the present invention.

[0063] An alternative estimate can be made by comparing the likelyperformance of the various embodiments described herein with the powerrequirements for the external blood warming apparatus presently known.Such external warming apparatus generally requires a supply of power onthe order of 1000-1500 watts and sometimes more. A device according tothe present invention will most likely require considerably less powerthan that. First, the present invention requires no external pump tocirculate the blood; this function is provided by the patient's ownheart. Accordingly, no power is needed to drive such a pump. Secondly,the present invention is considerably less complicated than externalblood warming systems. Known systems circulate the blood over arelatively lengthy path from the patient, through the warming element,and back into the patient. It is expected that more heat is lost overthis lengthy path than will be lost in any device according to thepresent invention.

[0064] Thus, the power required by external blood circulation andwarming systems of the type previously known can be used as a roughestimate of the likely upper limit for power required by a systemaccording to the present invention. It is most likely that such a systemwill best be equipped with a power supply having a capacity somewherebetween the two rough estimates described above. It is thereforecontemplated that a suitable power supply will be capable of providingpeak power somewhere in the range between 100 and 1500 watts, probablybeing in the range between 300 and 1000 watts. The ranges specified arean estimate of suitable peak power capability. The power supply willmost commonly be thermostatically controlled in response to atemperature sensor in the body of the catheter. The actual effectivepower transmitted to the patient will therefore typically be much lessthan the peak power capacity of the system power supply.

[0065] With respect to a catheter for cooling, the temperature and powerconstraints are not as limiting as is the case in a catheter for warmingblood. Care should merely be taken to avoid freezing the blood orinducing shock to the patient from too rapid cooling.

[0066] Blood is essentially water containing a number of suspended anddissolved substances. As such, its freezing point is somewhat below 0°C. However, a catheter adapted to cool blood in a hyperthermic patientor to induce an artificial hypothermia will usually not be operated attemperatures that low. It is presently contemplated that the externalsurface of such a catheter may be held in the range between about 20° C.and 24° C., although the actual temperature could vary between about 0°C. and the patient's current body temperature (somewhat in excess of 37°C.).

[0067] Various embodiments of apparatus suitable for practicing themethods of the present invention have been described. Other embodimentsand modifications will occur to those skilled in the art. For example,various means for heat transfer, e.g., resistance, including radiofrequency, heating; laser energy; pumped fluids; etc., may be combinedwith various means for increasing the effective heat transfer surfacearea, e.g., balloons, fins, ribs, etc., to optimize the function of adevice according to the present invention. Also, a temperature sensorwill typically be used although for ease of illustration such a sensoris not depicted in all of the embodiments described. Furthermore,although most of the figures depict embodiments in which only a limitedportion of the catheter is temperature controlled, no reason exists toprevent warming or cooling substantially the whole length of thecatheter.

[0068] Broadly stated, the present invention provides a method formodifying a patient's body temperature by controlling the temperature ofa catheter inserted into a blood vessel of the patient. Although severalillustrative examples of means for practicing the invention aredescribed above, these examples are by no means exhaustive of allpossible means for practicing the invention. The scope of the inventionshould therefore be determined with reference to the appended claims,along with the full range of equivalents to which those claims areentitled.

[0069] The present invention thus provides methods for both raising thebody temperature of initially hypothermic patients and lowering the bodytemperature of patients who are initially hyperthermic or for whom thebody temperature is to be lowered below normal for some other purpose.In all cases, it is possible that the target body temperature will beinadvertently exceeded due to an uncontrollable physiologic response ofthe patient, e.g., initially hypothermic patients may becomehyperthermic and initially hyperthermic patients may become hypothermic.In such cases, the present invention specifically provides for reversingthe heat transfer process so that patients passing into hyperthermia canbe immediately cooled and patients passing into hypothermia can beimmediately warmed. The control schemes for both warming initiallyhypothermic and cooling initially hyperthermic patients are set forth inFIG. 10. The initial, target, and overshoot temperatures for bothinitially hyperthermic and initially hypothermic patients are set forthin Table 1 below. TABLE 1 CONDITION Hypothermia Hyperthermia INITIALBODY Below 35° C. Above 38° C. TEMPERATURE TARGET BODY   36° C.   37° C.TEMPERATURE TARGET BLOOD 36.9° C. 36.9° C. TEMPERATURE OVERSHOOT   39°C.   36° C. TEMPERATURE

[0070] A preferred system for the selective warming and cooling ofpatients is illustrated in FIG. 11. The system comprises a catheter 100having a proximal end 102, a distal end 104, a heat-generating surface106 near the distal end, and a heat-absorbing surface near the distalend 108. The heat-generating surface 106 can be any of the heat transfercomponents described above, but will preferably be a wire coilresistance heater having from 50 to 1000 windings, typicallyspaced-apart from 0.1 mm to 1 mm. The total length of the catheter willtypically be from 15 cm to 50 cm, and the diameter will be from 1 mm to5 mm. Usually, the windings will extend over a total distance in therange from 10 cm to 20 cm near the distal end.

[0071] The exemplary heat-absorbing surface will be a thermallyconductive metal foil, typically composed of a biologically compatiblethermally conductive metal, such as gold, silver, aluminum, or the like.Copper will also be useful, but will have to be treated or encapsulatedin order to enhance biocompatibility. The foil will typically be thin inorder to enhance flexibility of the catheter body, typically having athickness in the range from 0.001 mm to 0.01 mm.

[0072] The heat-absorbing surface 108 will be conductively coupled to acooler located externally of the catheter, typically in a control unit120 as described below. in the illustrated embodiment, the surface 108is coupled by a thermally conductive core member 110 composed of aflexible rod or wire formed from one of the thermally conductive metalsdescribed above. Alternatively, thermal coupling can be achieved byextending the surface 108 proximally so that the proximal end of thesurface can be coupled to the cooler. In the latter case, it will bepreferable that the proximal portions of the surface 108 be thermallyinsulated to prevent cooling outside of the blood circulation.

[0073] The system will further comprise a control unit 120 whichtypically provides both the heat-generator and the cooler for couplingto the catheter 100. The heat-generator will usually comprise a directcurrent source for coupling to the resistance heater on the catheter.Usually, the direct current source will be a commercially available,temperature-controlled DC power supply, typically operating at a voltagein the range from 10 VDC to 60 VDC and a current output in the rangefrom 1 A to 2.5 A. Usually, the power supply will be controlled tomaintain the surface temperature of the heating surface 106 in the rangefrom 40° C. to 42° C. As discussed above, the surface temperature shouldnot exceed 42° C. in order to prevent damage to blood components. Otherdesirable characteristics of the heat exchange surface are describedabove.

[0074] Optionally, the temperature of the heat exchange surface can alsobe controlled based on measured blood temperature and/or measured bodytemperature. Blood temperature can be measured by temperature sensorspresent on the catheter. For example, a temperature sensor 112 may belocated on the catheter spaced-apart from the heat exchange surfaces 106and 108. The temperature sensor 112 may be located either upstream ordownstream from the heat exchange surfaces based on the direction ofblood flow and depending on the manner in which the catheter isintroduced to the patient. Optionally, a pair of temperature sensorscould be provided, one disposed on each side of the heat exchangesurfaces in order to measure both upstream and downstream bloodtemperatures. The catheter will also include a temperature sensor (notillustrated) coupled directly to the heat-generating surface 106 so thatthe temperature of the surface may be directly controlled. Othertemperature sensors (not illustrated) may be provided for directlymeasuring the patient's core body temperature, with the core bodytemperatures being fed back into the control unit 120.

[0075] The cooler in control unit 120 may be any type of refrigerationunit capable of removing heat from the heat-absorbing surface 106 at arate sufficient to cool the blood at a desired rate. Typically, thecooler will be rated at from 1 W to 100 W. Preferably, the cooler willbe a thermoelectric cooler, such as those commercially available fromMelcor Thermoelectrics, Trenton, N.J. 08648. The cooler will be directlycoupled to the core element 110 so that direct heat conduction from theheat-absorbing surface 108 may be effected to the cooler in control unit120. The temperature of the cooling surface 108 is less critical thanthat of the heating surface 106, but will usually be maintained in therange from 0° C. to 35° C. preferably being below 30° C. The temperatureof the cooling surface may be directly controlled within this range, oralternatively the system may be designed so that the cooling temperatureoperates approximately within this range based on the total systemcharacteristics.

[0076] The control unit 120 will further include one or more temperaturecontrollers for controlling the temperature of the heat-generatingsurface 106 and the heat-absorbing surface 106 based on the bloodtemperature and/or the body temperature. At a minimum, the control unit120 will provide for control of the temperature of the heat-generatingsurface 106 within the range set forth above, as well as for monitoringat least one of the patient blood temperature and patient bodytemperature in order to reverse the heating or cooling mode as discussedabove. In the exemplary embodiment, as described in FIG. 10, the controlscheme operates in an on-off mode, where for example hypothermicpatients are initially treated by warming the blood at a constantsurface temperature rate until a target temperature is reached. When thetarget temperature is reached, power to the heat-generating surface 106is turned off. Monitoring of the blood and/or patient body temperature,however, is maintained to assure that the patient temperature does notexceed a maximum which is above the target temperature. Should themaximum be exceeded, then the system is operated in the cooling modeuntil the excess body temperature is lowered. Usually, there will be noneed to again warm the patient, but the present system will provide forfurther cycles of warming and cooling if necessary. For initiallyhyperthermic patients, the cooling and warming modes are reversed.

[0077] It will be appreciated, for example, that the temperature controlschemes of the present invention could be substantially moresophisticated. For example, the power input to warm the patient could becontrolled based on proportional, derivative, or integral controlschemes which will typically provide for a tapering of the heat transferrate as the patient body temperature approaches the desired targetlevel. Moreover, cascade control schemes based on both patient bloodtemperature and patient body temperature could be devised. Such controlschemes, for example, could be adapted both for warming the patient andcooling the patient, with mathematical models of typical patientphysiological characteristics being taken into account in preparing thecontrol schemes. For the present, however, it is believed that a simpleoff-on control scheme with provision for reversing the heat transfermode if the target temperature is exceeded by more than a safe amountwill be sufficient.

What is claimed is:
 1. A system for restoring normal body temperature to a patient, said system comprising: an intravascular catheter having at least one heat transfer surface; a temperature sensor; and a control unit connectable to the temperature sensor and the catheter for selectively transferring heat to and from the at least one heat transfer surface to maintain normal body temperature.
 2. A system as in claim 1, wherein the catheter includes at least a heat-generating surface and a separate heat-absorbing surface.
 3. A system as in claim 2, wherein the heat-generating surface comprises a resistance heater and the heat-absorbing surface comprises a metal foil wrapped around the catheter.
 4. A system as in claim 3, wherein the resistance heater comprises a coil and the metal foil has an exposed area of at least 2 cm².
 5. A system as in claim 4, wherein the control unit comprises an electrical current source connectable to the resistance heater and a thermoelectric cooler connectable to the metal foil.
 6. A system as in claim 1, wherein the catheter includes at least one flow lumen which permits flow of a heat exchange medium past the heat transfer surface, and wherein the control unit includes a heater, a cooler, and a controller for selectively activating the heater or the cooler to heat or cool the heat exchange medium and restore normal body temperature to the patient.
 7. A system as in claim 6, wherein the heater is an electrical resistance heater and the cooler is a thermoelectric cooler.
 8. A system as in claim 1, wherein the temperature sensor is on the catheter and measures blood temperature.
 9. A system as in claim 1, wherein the temperature sensor is separately attachable to the patient to measure body temperature.
 10. A catheter for restoring normal body temperature to a patient by selectively transferring heat to or from blood flow, said catheter comprising: a catheter body having a proximal end and a distal end which is insertable into a blood vessel; a heat-generating heat exchange surface near the distal end of the catheter; and a heat-absorbing heat exchange surface near the distal end of the catheter.
 11. A catheter as in claim 10, wherein the catheter body has a length in the range from 15 cm to 50 cm and a diameter in the range from 1 mm to 5 mm.
 12. A catheter as in claim 10, wherein the heat-generating heat transfer surface comprises an electrical resistance heater and wherein the catheter further comprises a connector which connects the electrical resistance heat to an external current source.
 13. A catheter as in claim 10, wherein the heat-absorbing heat transfer surface comprises a metal foil wrapped around the catheter body.
 14. A catheter as in claim 13, wherein the metal foil extends from near the distal end to near the proximal end of the catheter body and wherein the proximal end of the foil is configured to engage an external cooler.
 15. A method for restoring normal body temperature to a patient having a body temperature above or below normal body temperature, said method comprising: selectively introducing heat to the blood flow for hypothermic patients or removing heat from the blood flow from hyperthermic patients; monitoring a temperature characteristic of the patient; and selectively removing heat through the catheter from the blood flow of initially hypothermic patients if the temperature characteristic indicates that the patient has or will become hyperthermic or introducing heat through the catheter to the blood flow of initially hyperthermic patients if the temperature characteristic indicates that the patient has or will become hypothermic.
 16. A method as in claim 15, wherein the heat is transferred via a catheter inserted into a blood vessel selected from the group consisting of the femoral artery, the jugular artery, and the jugular vein.
 17. A method as in claim 15, wherein the heat introducing steps comprise introducing heat at a rate between 10 W and 250 W.
 18. A method as in claim 17, wherein the heat introducing step comprises directing current through a resistance heater near the distal end of the catheter, passing radiofrequency current from the distal end of the catheter through the blood, circulating a heated medium through a heat exchanger near the distal end of the catheter, or directing light energy through a wave guide to the distal end of the catheter.
 19. A method as in claim 15, wherein the heat removing steps comprise removing heat at a rate between 1 W and 100 W.
 20. A method as in claim 19, wherein the heat removing step comprises (a) engaging a proximal end of the catheter against a cooler in order to conductively remove heat from a distal portion of the catheter along a heat conductive path on the catheter and to the cooler or (b) circulating a cooling fluid from the proximal end of the catheter, through a distal portion of the catheter, and back to the proximal end.
 21. A method as in claim 15, wherein the temperature characteristic monitoring step comprises monitoring at least one of body temperature and blood temperature.
 22. A method as in claim 15, wherein the surface temperature of the heating surface is maintained below 42° C.
 23. A method as in claim 15, wherein blood is being heated, wherein heating is stopped when the blood temperature reaches 36.9° C.
 24. A method as in claim 15, wherein blood is being cooled, wherein cooling is stopped when the blood temperature drops to 36.9° C. 